In a scanning electron microscope (SEM), a specimen is irradiated with an electron beam while scanning the electron beam. Then, a signal by means of electrons emitted from the specimen is converted into an intensity modulation input into a CRT, thus obtaining a scanned image (SEM image) of the specimen.
Japanese Patent Laid-open No. 7-192679 discloses that electrons emitted from a specimen are separated into secondary electrons and back scattered electrons so as to obtain the SEM image, thereby observing geometric and material information of the specimen with a higher resolution. Moreover, the Japanese Patent Laid-open No. 7-192679 discloses that, by using Wien filter type electromagnetic field which deflects the secondary electrons and the back scattered electrons from an optical axis of the electron beam, a detector is brought away from the optical axis, and thus the secondary electrons and the back scattered electrons are separated and detected effectively without exerting a bad influence on the electron beam.
The SEM is applied for an observation or a length measurement of a submicron order of contact hole or line pattern formed in a wafer specimen for a semiconductor device. In such an applied field, it is required that the SEM allow the observation to be made with a 10 nm or less of high resolution without charging a non-conductive material.
Generally speaking, a specimen for a semiconductor device is constructed by accumulating a non-electric conductive material such as SiO.sub.2 or SiN on a conductor part such as AL or Si. When such a specimen for a semiconductor device is irradiated with an electron beam, a surface of the non-electric conductive material becomes negatively charged (hereinafter, in some cases, this phenomenon is simply referred to as "charge up"), secondary electrons are produced from a bottom part of a contact hole. However, the secondary electrons produced from the bottom part of the contact hole are prevented from moving upwards by negative charges on the surface of the non-electric conductive material, and thus are not detected by a secondary electron detector. As a result, in some cases, there occurs an abnormal contrast or a tremendous distortion in the SEM image.
Such image troubles, which are attributed to the charge up on the surface of the specimen, result in an extreme difficulty in an observation of the contact hole or a length measurement of a line and space. This makes it difficult to estimate a semiconductor fabricating process, and in addition becomes a serious obstruction to ensuring quality of the semiconductor device itself.
A literature, "Material for 117th Study Session held by Japan Society for the Promotion of Science 132nd committee, Electron and Ion Beam Science and Technology", discloses that an employment of a low voltage SEM prevents the charge up on the surface of the specimen. If, however, an accelerating voltage for irradiating the specimen becomes low, energy of the electron beam is varied. This extremely decreases the resolution through chromatic aberration, thus making it difficult to embody a 10 nm or less of high resolution. Also, the accelerating voltage becomes low, less electron current flows. This extremely decreases a ratio between a secondary signal and a noise (S/N), brings about a worse contrast as the SEM image, thus making it difficult to make the observation with a high magnification and a high resolution. In particular, in a semiconductor device such as a one fabricated by an ultra micro fabrication technology, a signal produced from a bottom of trough of the contact hole or a line pattern is attenuated. This becomes a serious obstruction to making a precise observation or an accurate length measurement thereof.
Proposed in Japanese Patent Laid-open No. 5-266855 is a SEM according to a retarding method in which an electron beam is decelerated immediately before the electron beam attains to a specimen, thereby reducing the chromatic aberration so as to embody a high resolution and enhancing an extracting efficiency of electrons obtained from the specimen. In the SEM according to the retarding method, a positive electric voltage is applied to a liner tube in an electrooptic system so as to feed high energy to an electron beam passing through a lens position, and, by generating a decelerating electric field between the liner tube and specimen, the electron beam is decelerated immediately before the electron beam attains to the specimen.
Japanese Patent Laid-open No. 9-171791 discloses that, by locating an accelerating cylinder at an extended line of a passage through which an electron beam passes in an objective lens, an accelerating voltage is applied so as to accelerate the electron beam, and, by applying a superimposing voltage to a specimen, a decelerating electric field to the electron beam is formed between the accelerating cylinder and the specimen. Secondary electrons and back scattered electrons are absorbed into the accelerating cylinder by the decelerating electric field, and are detected by a secondary electron detector located above the accelerating cylinder.
Japanese Patent Laid-open No. 9-507331 discloses that two signal detecting systems are employed, one of which is a signal detecting system for forming an image of a surface of a specimen, and the other is a signal detecting system for forming an image of a bottom part of a scontact hole.